This disclosure is related to the field of electrical submersible pumps, (ESP) pump systems and methods for deployment of such pump systems in subsurface wells. More specifically, the disclosure relates to ESP deployment using an innovative arrangement where power is supplied to an ESP system using a tubing encapsulated cable (TEC) cable disposed in the ESP discharge fluid where the TEC is purposely operated at higher current densities than according to accepted electrical cable selection practices to minimize cable diameter, weight, cost, size of cable spooling equipment, complexity of the completion and subsequent capital costs.
The use of electric submersible pumps (ESPs) is well known to be advantageous in artificial lift of oil and gas from wellbores and for removing water (dewatering) gas wells, among other uses. Methods for deployment of ESPs, for example, on a small diameter threadedly connected jointed tubing (a conduit having a relatively small diameter to increase velocity of produced fluids to surface), requires the use of wellbore pipe lifting equipment such as a workover rig, and the cost of deployment can be significant, which in the case of smaller wells may inhibit exploitation of resources.
Part ‘rigless’ ESP deployment methods have been developed, including those using a downhole “wet” connect such that the ESP maybe deployed on a non-electrical cable and making electrical connection downhole using a special connector previously installed on the wellbore tubing, but such methods still require the tubing to be specially fitted out. Such fitment requires the use of a workover or other rig to prepare the well as does any failure of the downhole “wet” connect, cable and wellhead penetrator.
Deployment of retrofit ESPs on the power supply cable is believed to be desirable, however, such deployment has proven to be impractical using conventional ESPs and ESP cable, e.g., externally armored electrical cable. The ESP power supply cable transmits the required electrical power from a power supply to the ESP motor(s) disposed in a wellbore. ESP power supply cable is typically a specially constructed three-phase power cable designed specifically for use in subsurface well environments. The ESP power supply cable in ESP deployment methods and systems known in the art is banded or clamped to the exterior of the production tubing from below a surface control valve assembly coupled to the top of the well casing and production tubing (the “wellhead”) to the ESP system. Such cable is not designed to support its own weight.
A cable to be used for deployment of an ESP system must have adequate tensile strength to support its own weight, the weight of the ESP system, an allowance for overpull (tension applied to the cable in excess of the cable rated operating tension limit based on weight and depth plus the ESP system weight resulting from friction and other means by which the cable and ESP become lodged in the wellbore) and a safety factor.
Electrical conductor size in an ESP electrical power cable has a substantial effect on the external dimensions of the cable, the weight of the cable and its cost. The electrical conductor size is selected using design principles known in the art by determining the total amount of electrical current required to operate the motor(s) and any other electrically operated components of the ESP system substantially continuously, and using electrical equipment industry standard reference tables (examples set forth below) to select the appropriate electrical conductor size from among what are usually standard size electrical conductors. Typically the electrical conductor size is based on full load ESP motor running current, however, ESPs typically use induction motors in which case the motor starting current may be a factor of considerable significance in selection of the current carrying capacity (and resulting size) of the electrical power supply cable conductors.
One factor which is considered important in generating the above described industry standard reference tables for electrical conductors is to restrict electrical power losses in the cable due to electrical resistance. The normally accepted range is to restrict losses to the order of 2% to 5% of the amount of power supplied from the surface. One accepted standard is API Standard Recommended Practice (RP) 11S4, published by the American Petroleum Institute, Washington, D.C. API RP 11S4, which provides that a maximum of 5% voltage drop over the entire length of the cable from the power supply to the ESP will provide a reasonable operating efficiency. The voltage drop is related to the length of the cable, i.e., its depth in the wellbore, the resistance per unit length of the cable conductors and the total current drawn by the ESP system (whether at full running load or at starting load current). In conventional ESP installations, with a fixed, or limited voltage at surface, a long cable may cause such a voltage drop in the cable that there is insufficient voltage at the motor. Therefore, a larger conductor would be chosen. With a transformer in the surface electrical supply, the voltage at the surface end of the cable may be increased to compensate for the cable voltage drop, to retain adequate voltage at the motor. Therefore, the 5% voltage drop need not be a limiting factor.
In addition to power loss between the power supply and the ESP, which requires additional power from the surface power supply to provide the required electrical power at the ESP system, resistive losses cause heating of the electrical power supply cable. Excessive heating can cause the cable to deteriorate and eventually become unserviceable. To determine the allowable conductor temperature in its application, a power cable “ampacity” chart may be used (ampacity means ampere capacity, and is related to cable temperature).
IEEE Standard 1018-2013 ‘Recommended Practice for Specifying Electric Submersible Pump Cable—Ethylene-Propylene Rubber Insulation’ published by IEEE, 3 Park Avenue, N.Y. 10016-5997 U.S.A. provides guidance to determine the ampacity of an electrical cable for ESP use and includes standard reference tables.
Furthermore, because of the high cost of cable and installation, it is usual for the electrical cable conductor specification to be very conservative, that is, the electrical cable is selected to have a substantially greater ampacity than would otherwise be sufficient to carry the required electrical power to the ESP system from the surface. API RP 11S4 notes that using larger conductors will improve cable life by reducing internal heating caused by electrical current flowing in the cable.
The foregoing considerations may result in specification of a cable which is relatively large, complex, heavy and expensive. To provide abrasion resistance and tensile strength, electrical power cables known in the art have a plurality of small diameter steel or other high strength metal wire armor helically wound around the exterior of the cable. Such armor may limit the minimum allowable bend radius of the electrical power cable and may complicate sealing the electrical power cable where it passes through valves and related apparatus at the surface end of the wellbore (the “wellhead”) for connection to the surface power supply and related control system. To provide additional protection, some armored electrical cables include lead sheathing, for example, as explained in U.S. Pat. No. 5,414,217 issued to Neuroth et al. An electrical power cable with these characteristics is believed not to be suitable for use in connection with deployment apparatus such as the use of “wireline” well intervention and surveying equipment (including winches and pressure seals enabling the wireline to pass through the wellhead while maintaining a pressure tight seal).
Many devices are known in the art which address different aspects of the requirements of wellbore deployed electrical cables. For example, U.S. Pat. No. 5,086,196 issued to Brookbank et al. explains by way of background that cable-suspended ESP systems known prior to such patent require a specially constructed cable because conventional three-phase electrical power cable lacks sufficient tensile strength to support the weight of the ESP system. Such ESP electrical power cables known in the art prior to the present disclosure may have structural supporting members, as well as electrical conductors. Some of the electrical power cables known in the art were difficult to use and maintain because of the complexity of the cable construction, difficulty in splicing, and the tendency of the cable to rupture under gas depressurization. Early efforts in deploying ESP systems on an electrical power supply cable often resulted in cable failures and abandonment. More recently designed suspended electrical power supply cables have an even more complex cable utilizing molded vertebrae.
A further consideration concerns deployment of electrical apparatus such as a wellbore pump system into a “live” wellbore, that is, a wellbore in fluid communication with a fluid producing subsurface formation. At the surface connection (wellhead) in such wellbores, an electrical power cable is subject to a force which is related to the product of the wellbore fluid pressure at the wellhead and the cross sectional area of the wellbore power cable. Special measures have to be taken to withstand the forces resultant from the wellbore fluid pressure acting on the relatively large size of a conventional electrical cable, which may increase the cost and complexity of the installation.
Another problem encountered when using electrical cable for deployment of ESP systems is gas embolism due to rapid decompression of the cable after gases have dissolved in elastomeric materials used in the construction of the power cable. Rapid decompression may occur when the power cable is withdrawn from a well having substantial fluid pressure therein. One technique known in the art for addressing the embolism problem is to envelop the insulated electrical conductors of the power cable in a braid consisting of two layers of interwoven galvanized steel wires. Such cable construction has proven susceptible to kinking caused by thermal expansion of elastomeric electrical insulation and jacket material interacting with steel armor wires that surround the braid.
The Brookbank et al. '196 patent addresses another concern with wellbore-deployed electrical power cables, and describes an electro-mechanical cable for use in cable deployed pumping system which includes a containment layer surrounding a cable core and constructed to restrain outward radial expansion of the core while permitting longitudinal expansion.
There have been other approaches to simplify construction of an electrical power cable for use in a subsurface wellbore. For example, U.S. Pat. No. 4,928,771 issued to Vandevier discloses a system in which single-phase AC power is supplied from the surface along an insulated electrical conductor, with current return being along a wellbore casing. A phase converter converts the single-phase AC power to three-phase AC power downhole for driving the pump motor. This simplifies the cable, but requires downhole power electronics, which adds complication and risk of unreliability.
None of the foregoing electrical cables are designed for ESP system deployment using “wireline” winch equipment as they may have the following properties making them unsuitable for such deployment: the cables may be too heavy for a typical wireline unit winch; smaller, lighter cables may have insufficient tensile strength to carry the required load (cable weight, plus pump system weight, plus moving friction loss, plus tension changes due to tool manipulations in the wellbore); the minimum bend radius of cables having sufficient tensile strength may be too large for a typical wireline winch drum; and the minimum outer diameter of such cables may be too large to enable movement of the ESP system into a wellbore having fluid pressure at the surface when the wellbore is static (not flowing fluid). “Wireline” winch equipment is known in the art for deploying measurement and other types of electrically operated well intervention devices into subsurface wellbores at the end of an armored electrical cable. External diameters of such externally armored cables may be in a range of about 0.1 inches (6 mm) to about 0.5 inches (13 mm). Further, armored electrical cables known in the art including helically wound external armor wires necessarily have a rough exterior surface by reason such armor wires on the exterior surface, thus making them possibly unsuitable to make a long term wellhead pressure barrier which is required for a pump deployment.
This invention to deploy an electrical submersible pump using tubing encapsulated cable will overcome the difficulties explained above.